In the related art, in blood purification therapy, measurement of the arterial pressure (also referred to as the filter inlet pressure) and the venous pressure during extracorporeal circulation is carried out with a pressure monitor line provided on an air trap chamber in an extracorporeal circuit connected to respective pressure sensors. In this method, the pressure is measured via air, and hence adjustment of a liquid level in the chamber is required when priming is completed or during the treatment in order to provide an air space. The adjustment of the liquid level is carried out by pushing and pulling a syringe, which is connected to a distal end of a liquid level adjustment line provided separately in the air trap chamber. However, this procedure requires a certain level of skill, and if the procedure is conducted improperly, blood may jet out and come into contact with a human body or a pressure sensor protecting filter, whereby a high risk of infection results.
Therefore, various blood pressure measurement devices including a pipe and at least one section for measuring the pressure of blood flowing in the pipe are proposed. These devices include a substantially hard wall and a hole sealed by a closing member which can be resiliently deformed or displaced, and the inner surface of the pipe comes into contact with blood and the outer surface thereof comes into contact with the outside air so that a force exerted on the closing member can be measured by a load sensor. For example, such blood pressure measurement devices include (1) a device in which an air compartment is provided between a thin film (or diaphragm) and a load sensor so as to measure directly the internal pressure in the air compartment which varies with displacement of the thin film, (2) a device in which a force exerted on the inner surface of a flexible thin film, which is provided for sealing a hole provided on a wall of the compartment, is transmitted to a load sensor via a load transmitter, to be measured, (3) the device in (2), in which a metallic disk is mounted to the outer surface of the flexible thin film, and a magnet is attached to the end of the shaft of a load transmitter, so that the flexible thin film is fixed to the load transmitter, and (4) a device in which a closing member is formed in a single member which includes a hard wall of a blood pressure measurement section. See JP-A-2002-233570 (FIG. 1, FIG. 2, FIG. 3, and FIG. 6).
However, the device in (1) described above has a drawback such that the seal of the air compartment may be broken during use, and when the seal is broken, the pressure transmitting function may be lost. The devices in (2) to (3) described above have a drawback in that the hole on the wall must be sealed completely with the thin film, and hence mounting of the thin film onto the wall of the compartment is relatively complicated, whereby manufacture and assembly of the devices are difficult. The device shown in (4) also has a drawback in that the section which can be resiliently deformed is formed to have a waveform outer shape, whereby manufacture and assembly of the device are also difficult. All of the devices (1) to (4) described above include an inner wall of a pressure measuring section, which is larger than the inner wall of a liquid circulation circuit, and whose shape is square in cross-section. Therefore, when it is applied to a blood circulation circuit, eddy flow occurs in blood flowing into the pressure measuring section, and hence there is a risk that plaque formation may result.